This invention relates to improved cation exchange membranes and to methods for their production. The invention is further directed to methods for the electrolysis of an aqueous solution of an alkali metal halide by use of these cation exchange membranes, and to the electrolytic cells in which the electrolysis takes place.
It has been known to the art to obtain a cation exchange membrane of a perfluorocarbon polymer containing pendant sulfonic acid groups by saponification of a membrane prepared from a copolymer of tetrafluoroethylene and perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonyl fluoride. This known perfluorocarbon type cation exchange membrane containing only sulfonic acid groups, however, has the disadvantage that the membrane, when used in the electrolysis of an aqueous solution of an alkali metal halide, tends to permit penetration therethrough of hydroxyl ions back migrating from the cathode compartment because of the high hydrophilicity of the sulfonic acid group. As a result, the current efficiency during electrolysis is low. This is a special problem when the electrolysis is used for the production of aqueous solutions of caustic soda at concentrations of more than 20 percent. In this reaction, the current efficiency is so low that the process is economically disadvantageous compared with electrolysis of aqueous solutions of sodium chloride by the conventional mercury process or diaphragm process.
The disadvantage of such low current efficiency can be alleviated by lowering the exchange capacity of the sulfonic acid group to less than 0.7 milliequivalent per gram of the H form dry resin. Such lowering, however, results in a serious decrease in the electroconductivity of the membrane and a proportional increase in the power consumption. This solution, therefore, is not without its economic difficulties.
U.S. Pat. No. 3,909,378 discloses composite cation exchange membranes containing sulfonic acid moieties as the ion exchange group and comprising two polymers with different equivalent weight (EW), that is number of grams of polymer containing one equivalent weight of ion exchange functional group. When such membranes are utilized in the electrolysis of aqueous solutions of sodium chloride, high current efficiencies are obtained by effecting the electrolysis with the higher EW polymer side of the composite membrane facing the cathode. For high current efficiency coupled with low power consumption, the value of EW of the higher EW polymer must be increased and the thickness decreased as much as possible. It is, however, extremely difficult to produce a composite cation exchange membrane having a current efficiency of not less than 90 percent by use of membranes containing only sulfonic acid groups.
In U.S. Pat. No. 3,784,399, German Pat. OLS No. 2,437,395 (U.S. Patent Application Ser. No. 406,361 filed Oct. 15, 1973) and German Patent OLS No. 2,447,540 (U.S. Patent Application Ser. No. 425,079 filed Dec. 17, 1973), there are suggested cation exchange membranes wherein the cathode side surface layers of fluorocarbon cation exchange membranes contain sulfonamide groups, salts thereof or N-mono-substituted sulfonamide groups. These membranes, however, are deficient in electrochemical and chemical stabilities.
An object of this invention is to provide fluorocarbon cation exchange membranes which, even in electrolysis for production of caustic soda at high concentration, more effectively inhibit the back migration of hydroxyl ions and enable the electrolysis to proceed constantly with higher current efficiency than the conventional fluorocarbon cation exchange membranes and to provide a method for the manufacture of said membranes.
Another object of this invention is to provide a method for the electrolysis of the aqueous solution of an alkali metal halide by use of such cation exchange membranes.